Conventionally, piezoelectric sensors have been employed in various fields.
As an example, there is a transparent input device using a piezoelectric sensor. Examples of the transparent input device in use are a resistance-film transparent input device (Japanese Laid-Open Publication No. 242759/1993 (Tokukaihei 5-242759; published on Sep. 21, 1993)), a capacitance transparent input device (Japanese Laid-Open Publication No. 324203/1993 (Tokukaihei 5-324203; published on Dec. 7, 1993)), an analog-capacitor transparent input device, an ultrasonic-surface-elastic-wave transparent input device (e.g., Japanese Laid-Open Publication No. 182842/2002 (Tokukai 2002-182842; published on Jun. 28, 2002)), and an infrared-scanning transparent input device (e.g., Japanese Laid-Open Publication No. 45155/1999 (Tokukai 11-45155; published on Feb. 16, 1999)).
The resistance-film transparent input device is constituted of a pair of substrates opposite to each other, i.e., a top sheet (upper substrate) on a surface of an input panel and a lower substrate opposed thereto. Further, an inside of the upper substrate is coated with a transparent conductor film, and an inside of the lower substrate is also coated with a transparent conductor film. The transparent conductor films are opposed to each other at a predetermined distance. That is, the transparent conductor films are out of contact with each other.
Meanwhile, the capacitance transparent input device and the analog-capacitor transparent input device, as with the resistance-film transparent input device, detect a contact position based on such a principle that a capacitance changes when a finger touches an input panel coated with a transparent conductor film layer.
Further, the ultrasonic-surface-elastic-wave transparent input device and the infrared-scanning transparent input device detect a contact position by scanning a surface of a transparent panel with a surface elastic wave and infrared light respectively.
In case of the resistance-film transparent input device, when the top sheet is subjected to outside pressure, the transparent conductor films disposed at a distance from each other are caused to come into contact with each other. The contact position is calculated from a voltage gradient of the transparent conductor films for detection. Therefore, in the resistance-film transparent input device, the top sheet needs to be deformed by being subjected to outside pressure. As a result, the resistance-film transparent input device raises such problems that: the transparent conductor films are flawed by coming into contact with each other, and durability is impaired because the transparent conductor films need to be deformed.
Meanwhile, the capacitance transparent input device and the analog-capacitor transparent input device detect a contact position according to a capacitance change. Therefore, the capacitance transparent input device and the analog-capacitor transparent input device raise such problem that they may malfunction when an electromagnetic noise is generated.
Moreover, the ultrasonic-surface-elastic-wave transparent input device and the infrared-scanning transparent input device raise such problem that they tend to have a complex structure and have difficulty in dealing with simultaneous multipoint contact.
Accordingly, a piezoelectric sensor and a transparent input device have been demanded which have an excellent durability and an anti-noise property.
Further, as an example of other applications of a piezoelectric sensor, there is a piezoelectric sensor which is installed in a high-temperature structure such as a pipe and a valve in a plant (e.g., an atomic power plant) or an internal-combustion engine in order to detect an abnormality in the structure. For example, an acoustic emission sensor and a piezoelectric oscillation sensor have been used. The acoustic emission sensor detects an acoustic emission, i.e., an elastic wave which is generated when the structure is cracked and broken. The piezoelectric oscillation sensor detects an abnormal oscillation and information on acceleration. These sensors come in various types, such as a compression type, a cantilever type, a diaphragm type, and a shear type.
Among them, a compression-type thin-film piezoelectric sensor including: a laminated body having a pedestal with a pedestal-side electrode; a piezoelectric body; a load-body-side electrode; and a load body, wherein these materials are laminated in this order, and the compression-type thin-film piezoelectric sensor is used with a lower surface of the pedestal strongly, i.e., firmly mounted on a target object. When an oscillation occurs in the target object, the oscillation is transmitted to the pedestal side of the sensor. Whereas the pedestal side of the sensor oscillates together with the target object, the load body side oscillates with delay due to inertial force, and the piezoelectric body is subjected to a compressive stress or tensile stress proportional to oscillatory acceleration. Further, a potential or a voltage proportional to the stress is generated on both sides of the piezoelectric body and is extracted (taken out) by the two electrodes (the pedestal-side electrode and the load-body-side electrode). A measurement of the electrical output so extracted makes it possible to detect a size of the oscillation or the acceleration of the target object.
Conventionally, a piezoelectric body made of a piezoelectric material such as lead zirconate titanate and vinylidene polyfluoride, as described in Japanese Laid-Open Publication No. 148011/1994 (Tokukaihei 6-148011; published on May 27, 1994) and Japanese Laid-Open Publication No. 206399/1998 (Tokukaihei 10-206399; published on Aug. 7, 1998), has been used for such a piezoelectric sensor. However, a piezoelectric body made of such a piezoelectric material has a low Curie point (the term “Curie point” means a temperature at which a polarization of such a piezoelectric body disappears.) and therefore has a maximum operating temperature of about 300° C. at the highest. Accordingly, in order to keep a temperature of a piezoelectric body at an applicable temperature, Japanese Laid-Open Publication No. 203665/1993 (Tokukaihei 5-203665; published on Aug. 10, 1993) discloses a piezoelectric body cooled with a peltiert element. However, because the peltiert element only has a function of simply generating a local temperature gradient, a cooling mechanism cannot be mounted on an outside remote from the piezoelectric body, so that the peltiert element cannot be applied to a part where a whole of the piezoelectric body becomes hot.
Therefore, as described above, because the conventional thin-film piezoelectric sensor cannot withstand high temperatures, an oscillation of a target object which reaches a high temperature is brought through an oscillation transmission bar to a remote low-temperature environment for measurement. However, an oscillation such as an acoustic emission is attenuated due to a property of an oscillation transmission substance in the process or is mixed with an external redundant oscillation in the process of transmission, so that the oscillation of the target object cannot be measured sufficiently accurately. That is, for the purpose of an accurate measurement, it is desirable that an oscillation be measured in a place as proximate as possible to the place where the oscillation has occurred.
This is achieved by a thin-film piezoelectric sensor, disclosed in Japanese Laid-Open Publication No. 34230/1993 (Tokukaihei 5-34230; published on Feb. 9, 1993), which withstands high temperatures and whose piezoelectric layer is made of a piezoelectric material such as lithium niobate, which has a high Curie point. Lithium niobate has a Curie point of about 1140° C. and can be used in a high-temperature environment without cooling means. However, lithium niobate is hard to make thinner and needs to be a monocrystalline body to obtain a piezoelectric property, thereby raising such problem that it is difficult to produce and process the sensor at low cost.
A high-temperature thin film oscillation sensor described in Japanese Laid-Open Publication No. 122948/1998 (Tokukaihei 10-122948; published on May 15, 1998), in order to solve these problems, is arranged so that zinc oxide or aluminum nitride is used as a piezoelectric ceramic having no Curie point, and a thin film including the piezoelectric ceramic oriented in a c-axis direction is used as a piezoelectric thin film element.
However, a substance whose crystal has a wurtzite structure (e.g., zinc oxide and aluminum nitride, described in the foregoing patent document) has difficulty in retaining a piezoelectric property, and it is impossible to stably improve a piezoelectric property only by a c-axis orientation of an axis of the crystal. That is, a c-axis orientation is a factor necessary to improve a piezoelectric property but is not sufficient by itself to stably retain a piezoelectric property. Experiment data shows that even when a substance having an excellent piezoelectric property can be produced, the substance is not reproducible. In some cases, a piezoelectric property is not expressed at all.
This is because a piezoelectric sensor produced by the method of the foregoing patent document has a substrate and a piezoelectric layer provided directly thereon and therefore cannot stably align a direction of a dipole of a crystal of a piezoelectric element. Even when a piezoelectric element which has a high dipole orientation degree is produced, it is difficult to obtain a piezoelectric element whose piezoelectric layer has a high dipole orientation degree. Specifically, it is impossible to cause the piezoelectric layer to keep a dipole orientation degree not less than 75%. This prevents the piezoelectric sensor from retaining a piezoelectric property and causes such problem that pressure cannot be detected satisfactorily.
Accordingly, a small, inexpensive thin-film piezoelectric sensor for detecting an acoustic emission and an oscillation or acceleration has been demanded which ensures a piezoelectric property by thinning a piezoelectric material having no Curie point and orienting a polarity of a crystal in the thin film, requires no cooling means, and has an excellent durability.
Moreover, such conditions are required in a cylinder internal-pressure sensor for grasping a phenomenon in a combustion chamber of an internal combustion engine. Conventionally, the cylinder internal-pressure sensor, disposed on an inner surface of a cylinder, transmits internal pressure of the cylinder through a diaphragm and a pressure transmission bar to a piezoelectric element, and extracts from the piezoelectric element an electrical signal proportional to a size of the internal pressure of the cylinder, so that the pressure is detected. The foregoing piezoelectric element is generally a piezoelectric element made of a ceramic material such as lead zirconate titanate and lead titanate.
However, as with an ignition plug, a piezoelectric sensor which directly measures internal pressure of a cylinder is exposed to a high combustion temperature (500° C.), and a piezoelectric element reaches a very high temperature (about 400° C.).
A ceramic piezoelectric element made of lead zirconate titanate has a Curie point of about 250° C.; a ceramic piezoelectric element made of lead titanate has a Curie point of about 350° C. These temperatures are both lower than the foregoing combustion temperature and undesirably allow the piezoelectric elements to reach their respective Curie points. When a piezoelectric material reaches a high temperature exceeding a Curie point, a piezoelectric element exhibits a deterioration in a piezoelectric property due to depolarization and the like and therefore becomes unusable, so that the piezoelectric element is usually used in combination with separate cooling means for keeping a temperature of the piezoelectric element at a suitable temperature.
Meanwhile, an arrangement requiring no cooling means may be achieved by a piezoelectric element disclosed in Japanese Laid-Open Publications No. 34230/1993, as described already, and Japanese Laid-Open Publication No. 180286/2000 (Tokukai 2000-180286; published on Jun. 30, 2000). The piezoelectric element is made of a monocrystalline piezoelectric material (e.g., lithium niobate) which has a relatively high Curie point. Lithium niobate has a Curie point of about 1140° C. Therefore, even when the piezoelectric element reaches a high temperature of about 400° C. in case of measuring internal pressure of a cylinder, the piezoelectric element, having a much higher Curie point, does not deteriorate, thereby requiring no cooling means.
However, lithium niobate, having a low processability, is hard to make thinner and needs to be used in a monocrystalline state. Moreover, because a special method is required to form lithium niobate into an arbitrary shape, lithium niobate is limited in handling and therefore causes a problem with cost.
Further, lithium niobate has a problem with retention of a monocrystal. When a monocrystal of lithium niobate is brought into direct contact with a diaphragm and the diaphragm is subjected to uneven pressure, an electrode, disposed on an opposite side of the diaphragm, which serves to retain the monocrystal, is distorted. In the worst case, a retention part may be damaged. In order to prevent this, a bar-like pressure transmission mechanism for transmitting internal pressure of a cylinder to a piezoelectric element is required. However, this inevitably results in a complex structure.
For example, Japanese Laid-Open Publication No. 34230/1993 discloses a pressure sensor which has a detection element and a diaphragm. The detection element is constituted of a piezoelectric element, a pressure transmission mechanism, and the like, and is stored in an inside of a main metal body mounted in a sensor-mounting screw hole provided in a cylinder block. Also, the diaphragm is press-fitted onto a lower end surface of the main metal body facing a cylinder. However, a pressure transmission bar needs to be provided between the diaphragm and the piezoelectric element.
Further, in Japanese Laid-Open Publication No. 180286/2000, a pressure transmission bar is not used, but a diaphragm is provided with a projection. A piezoelectric element has a load-receiving structure which generates a compressive stress so that a pressure detection element is not deflected (bent) even under a load due to internal pressure of a cylinder from the projection of the diaphragm.
Thus, the conventional piezoelectric materials cause a pressure transmission structure to be complex, large and expensive and therefore cannot satisfy demand.
Accordingly, in view of this, an inexpensive piezoelectric sensor having an excellent durability and a simple structure has been demanded.
The present invention, completed in consideration of the foregoing problems, has a first object to provide a piezoelectric sensor, made of a transparent pressure-sensitive material having a piezoelectric property, which has an excellent durability and an anti-noise property, and a transparent input device including the same.
Further, the present invention has a second object to provide a small, inexpensive piezoelectric sensor, ensuring a piezoelectric property, requiring no cooling means, and having an excellent durability, which detects an acoustic emission and an oscillation or acceleration, or detects internal pressure of a cylinder in order to grasp a phenomenon in a combustion chamber of an internal combustion engine.